It has been proposed to reduce noise on the video signal from an imager by integration of a number of frames using a frame store. For example in one known arrangement (FIG. 1) a frame store 1 is connected in a recursive digital filter arrangement and receives a digitised video input. The digits that are read out of the frame store are delayed by one frame relative to those written in, and so digits representing pixels of any particular line of a given frame appearing at the input have subtracted from them digits representing respective pixels of the corresponding line of the previous frame. This difference signal is fed to a PROM k configured as a look-up table, the output of which may be attenuated compared to the input, and the output of the look-up table is added to the output of the frame store to produce the video output. A manual control enables one of the characteristics shown in FIG. 2 for the PROM k to be selected. The characteristic k.sub.1 (k or kernel=1) corresponds to no integration, while the characteristic k.sub.4 corresponds to maximum integration. It will be seen that, in case of no integration, the video output is equal to the video input, since the input has the output of the frame store both subtracted from it and added to it. In the case of k=4, corresponding pixels for any line are averaged over a number of rames in the video output. The manual control is adjusted to accommodate different levels of random noise corresponding to different scene conditions. In the case of a noise impulse on a single pixel in a single frame, this will be averaged to a lower and less obtrusive value over several frames. Clearly, however, integration will also have the undesired effect of averaging the position of an object that changes its position between several frames, so-called motion smear,
To mitigate such impairment of dynamic resolution, it has been proposed (GB-A-1515551) to make the value of k vary with signal level i.e. noise reduction at level k=4 only takes place when the input to the PROM 2 (the difference between the video input and the video output) is below a certain threshold (FIG. 3): it is assumed that small variations between successive video frames stem from noise while larger variations stem from motion. Consequently, the pixels in a line which were very similar in value to those in that line in previous frames, would be subject to noise reduction, whereas those in a line which differed significantly from those in the same line in preceding frames, would not. However, the threshold needs to be set above the level of fluctuations to the difference signal that is caused by noise, but the level of noise can itself vary.
The applicants considered constructing an adaptive system (FIG. 4) in which several different characteristics (FIG. 5) could be selected dependent on the magnitude of the difference signal (between the output and the input to the noise reduction circuit) and hence dependent on the level of noise (but averaged over many frames). Differences which would correspond to a good signal to noise ratio e.g. better than 39 dB (band a) produce a characteristic A, differences which would correspond to a medium signal to noise ratio lying in the next band b(33 db-39 dB) produce a characteristic B, those in a next band c (27 dB-33 dB) would result in a characteristic C, differences corresponding to a poor signal to noise ratio, worse than 27 dB (band d) would produce characteristic D. The signal resulting from the difference between each new incoming frame and the output of the frame store which would be dependent on the average of several recent frames passes through a rectifier 3 and low pass filter 4 before being converted in an analogue-to-digital converter 5 to one of a number of codes controlling PROM 2. Thus, for example, if the signal to noise ratio lay within band b over several frames, difference signals to PROM 2 below the threshold would be attenuated, those above the threshold would not. If the signal to noise ratio then became worse, the higher threshold of characteristic C would apply.
However, such an arrangement would be prone to error since the measurement of signal to noise ratio would be by comparing successive frames and any change due to motion would make the result inaccurate. For this reason, it has been proposed for the control of noise reduction in a noise reduction circuit to be dependent on the gain of a stage of the imager, which gain is related to the noise generated by the imager (EP-A-0328346 and U.S. Pat. No. 4,851,911).